1. Field of the Invention
The invention relates to a method for calculating an absolute time difference in a radio system, and to a radio system.
2. Description of the Related Art
In radio systems, what is known as absolute time (AT) is usable as a reference time for various purposes. One of the most current applications is the use of absolute time in positioning.
Positioning a subscriber terminal, i.e. determining its geographical location, has become an increasingly important function in cellular radio networks. For instance in the United States, the Federal Communication Commission (FCC) requires that all subscriber terminals making an emergency call be positioned with an accuracy of up to 50 meters. Positioning can be also utilized for commercial purposes, e.g. for determining different tariff areas, or for implementing a navigation service guiding a user or simply for positioning family and friends.
Various methods are used for implementing the location service (LCS). Roughly, the location of a subscriber terminal can be positioned based on the identity of the cell serving the subscriber terminal. This is not very accurate information, since the diameter of a cell may be tens of kilometers.
A more accurate result is obtained by using radio link timing information as additional information, for instance the timing advance (TA). In the GSM system (Global System for Mobile Communications), the TA specifies the location of a subscriber terminal with the accuracy of about 550 meters. The problem is that if the cell is implemented with an omnidirectional antenna, then only the location of the subscriber terminal in known relative to a base transceiver station on a circle drawn around it. For example, a base transceiver station sectored into three sections improves the situation to some degree, but also in this case the location of a subscriber terminal can be positioned only in a 120-degree sector in a 550-meter deep area at a given distance from the base transceiver station.
These inaccurate methods are sufficient for some applications, e.g. for determining tariff areas. Methods that are more accurate have also been developed. Uplink methods are based on several different base transceiver stations making measurements from a signal transmitted by a subscriber terminal, an example being the TOA method (Time of Arrival).
However, downlink methods are more common, mainly due to a better capacity. In these, a subscriber terminal makes measurements from signals transmitted by several different base transceiver stations. An example of such a method is the E-OTD method (Enhanced Observed Time Difference). Since, in practice, a radio network is never completely synchronic, the actual timing of signals transmitted by base transceiver stations has to be measured. This can be taken care of by using a location measurement unit (LMU), placed in a measuring point having a known location. The location measurement unit serves to determine the real timing difference between the transmissions of the base transceiver stations. The effect of the real timing differences is eliminated from the results measured by the subscriber terminal, after which the location of the subscriber terminal can be determined geometrically based on the coordinates of the base transceiver stations, e.g. at the incidence point of the hyperboles or circles descriptive of the propagation time delays. In WCDMA systems (Wideband Code Division Multiple Access), the corresponding method is called the IPDLTOA method (Idle Period Down Link Time Difference Of Arrival).
In positioning systems, in most cases, the time difference between the clocks of base transceiver stations is determined by using what is called their real time difference (RTD), which are determined based on signals received by a location measurement unit from the base transceiver stations. However, base transceiver station time difference determination based only on RTD specifications requires much calculation capacity. In fact, positioning methods, for instance the E-OTD method, can also be applied by using what is known as the absolute time (AT) and absolute time differences (ATD), by means of which the need for calculation capacity is reduced. The use of the absolute time can be implemented for instance by determining the absolute time of what is known as a reference base transceiver station (Reference AT) relative to the time determined by using the receiver of a satellite positioning system. Typically, this is implemented relative to what is known as GPS time, determined using a GPS satellite positioning receiver (Global Positioning System), allowing the GPS receiver to be located for instance in a location measurement unit.
However, up to now, the problem in using absolute time was more error factors in the use of absolute time compared with the use of only RTD. Error in RTD specifications is caused for instance by the digital signal processing process (channel model error, arrival time of DSP signal) (Digital Signal Processing, DSP), and multipath propagation. When AT specifications are used, the error caused by for instance the clock oscillator and the GPS has to be added hereto. In methods known up to now, the aim has been to reduce the timing error by improving the accuracy of the GPS measurement, which has turned out to be difficult. In addition, GPS measurement is, however, only one factor in the AT specification error.
The object of the invention is to provide an improved method for calculating the absolute time difference in a radio system, and an improved radio system.
The improved method comprises maintaining location measurement units having a known location in the radio system and location measurement areas specified thereby, the location measurement units receiving signals from base transceiver stations in their location measurement area, one base transceiver station in the location measurement area being a location measurement unit reference base transceiver station, and at least one measured base transceiver station in the location measurement area being common to a second location measurement area; specifying a reference time for the reference base transceiver station and the detected time differences of the base transceiver stations in the location measurement area relative to the reference base transceiver station; reporting the reference time of the reference base transceiver station of the location measurement area and the detected time differences of the other base transceiver stations in the location measurement area for updating a base transceiver station absolute time difference table maintained in the radio system; calculating a computational absolute time for the reference base transceiver station by using the absolute time difference in the absolute time difference table of at least one reported base transceiver station, the difference being affected by the report of another location measurement unit, and the detected time difference reported by the location measurement unit to this base transceiver station; calculating a corrected absolute time for the reference base transceiver station by using the computational absolute time specified for it, the reported reference base transceiver station reference time and a correction coefficient; and using the corrected absolute time of the reference base transceiver station and the detected time differences of the reported base transceiver stations for calculating the absolute time differences for the base transceiver stations, and storing the computational absolute time differences of the base transceiver stations in the absolute time difference table.
The improved radio system comprises at least one subscriber terminal to be located, and at least three base transceiver stations used in the locationing and having a known location, one of the base transceiver stations operating as the base transceiver station serving the subscriber terminal. The improved radio system also comprises: at least two location measurement units having a known location and location measurement areas specified by them, the location measurement units receiving signals from base transceiver stations in their location measurement area, one base transceiver station in the location measurement area being a reference base transceiver station of the location measurement unit, and at least one measured base transceiver station being common to a second location measurement area; means for calculating an absolute time difference in the radio system locationing method, a location service absolute time difference table for storing and maintaining absolute time difference values of base transceiver stations; means for specifying a reference base transceiver station reference time; means for specifying detected time differences of the base transceiver stations in the location measurement area relative to the reference base transceiver station; means for maintaining values of the absolute time difference table of the base transceiver stations of the radio system; means for reporting the reference time of the reference base transceiver station of the location measurement unit and the detected time differences of the other base transceiver stations in the location measurement area for updating the base transceiver station absolute time difference table maintained in the radio system; means for calculating a computational absolute time for the reference base transceiver station by using the absolute time difference in the absolute time difference table of at least one reported base transceiver station, the difference being affected by the report of another location measurement unit, and the detected time difference reported by the location measurement unit to this base transceiver station; means for calculating a corrected absolute time for the reference base transceiver station by using the computational absolute time specified for it, the reported reference base transceiver station reference time and a correction coefficient; and means for using the corrected absolute time of the reference base transceiver station and the detected time differences of the reported base transceiver stations for calculating the absolute time differences for the base transceiver stations, and means for storing the computational absolute time differences of the base transceiver stations in the absolute time difference table.
Other preferred embodiments of the invention are described in the dependent claims.
The invention is based on using absolute time and absolute time differences of base transceiver stations in the positioning. In the radio system, a base transceiver station absolute time difference table, i.e. an ATD table, is maintained and updated based on reports from the location measurement units. The effect of the timing of the base transceiver stations is compensated for by means of the absolute time differences, i.e. the ATD values, when calculating the location of a subscriber terminal based on the OTD results (Observed Time Difference) reported by it. The report of the location measurement unit includes a reference base transceiver station reference time (Reference AT) and detected time differences of the base transceiver stations, i.e. OTD:s. In the method of the invention, the error caused by the measurements is corrected by correcting the results of a new report to be placed in the ATD table by suitable weighting in the direction of the expected absolute time differences of the ATD table to minimize the relative error between the location measurement areas.
The method and radio system of the invention bring forth a plurality of advantages. The calculation method of the invention enables a reduction of the relative error caused in the value of the absolute time of a base transceiver station, since each measured value is corrected in accordance with the results of several location measurement units. Another advantage of the invention is that it enables the use of absolute time, which requires less calculation capacity.
A further advantage of the method is that its use enables a pre-correction, before or during the calculation of the absolute time difference, of the error caused by other delays, e.g. cable or reflection delays or delays caused by the height of the measurement antenna.